Even though the most rapid growth rates which occur in the human are during fetal life, the hormonal mechanisms for the control of fetal growth have not yet been defined. Of the known hormones which exhibit anabolic effects, only immunoreactive insulin appears to be of any possible importance in the stimulation of fetal growth. The proposed project is based on the hypothesis that the primary stimulators of fetal growth are insulin-like peptide hormones. The objectives of the project are therefore to quantitate the levels of growth promoting hormones in fetal and maternal plasma, to identify the hormones controlling the production of insulin-like growth promoting hormones and to determine the site of production of those hormones which stimulate growth of the fetus, to quantitate the metabolic responses of fetal tissues to the insulin-like growth promoting peptides, and to assess the physical interaction of the insulin-like growth promoting hormones with the plasma membrane receptors of fetal tissues. Methods to be used in this study will include measurement of hormones in fetal plasma and tissue extracts by the radioimmunoassay for insulin, growth hormone, human chorionic somatomammotrophin, and somatomedin; receptor binding assays for insulin and somatomedin; studies of the production of growth promoting peptides by known hormones in fetal tissues; and assessment of the interaction of insulin-like growth promoting peptides with fetal tissue plasma membrane receptors.